An austenite stainless steel with excellent corrosion and heat resistance is utilized as important pipe arrangement materials in a wide range of industrial field such as steam-power generation, atomic power generation, automotive engineering, petrochemistry and, chemical engineering.
Recently, austenite stainless steel pipes with high nickel content are being noted especially as compressed hydrogen gas storage tank materials to be employed in a fuel cell vehicle (FCV) that is regarded as the most likely candidate car, among eco-friendly cars, immediately following an electric vehicle (EV) which has already been put to practical use.
The reason why austenite stainless steel pipes with high nickel concentration are noted as the aforementioned tank materials is due to enduring hydrogen gas embrittlement (HGE) under compressed hydrogen gas environment, while most metal pipe materials tend to cause HGE in such hydrogen gas environment.
However, application of conventional austenite stainless steels having high nickel concentration such as SUS316L and SUS310S to the compressed hydrogen gas storage tank material will be impossible, because of their low strength.
Therefore, such high nickel austenite stainless steel pipes have to be largely strengthened without loss of their ductility in order to employ them as the hydrogen tank.
As nitrogen (N) in an amount of, e.g., about 0.9% (by mass) is added to a chromium-nickel type stainless steel having a composition equivalent to that of SUS316L which is typical high nickel austenite stainless steel, the resulting stainless steel increases in offset yield strength (yield strength) to about three times as high as that of SUS316L stainless steel, with no decrease in fracture toughness yet with much more improvements in corrosion resistance, especially in pitting corrosion resistance.
So far, high-N austenite steels having nitrogen in an amount of up to about 0.1 to 2% (by mass) have been produced by melting-solidification processes usually in nitrogenous atmospheres.
In such melting solidification processes, however, a large amount of nitrogen gas liberates during solidification of the liquid phase due to the nitrogen solubility gap between both phases of liquid and solid, leading to formation of faults like blow holes in the solidified products. Moreover, there are difficulties that segregation generating in the solid phase cannot be avoided especially in large solid products. Accordingly, sound products without such faults are difficult to obtain by the melting-solidification processes as mentioned above.
Now, a high-nitrogen austenite stainless steel material, too, has been intensively tried to manufacture by a nitrogen-absorption and solid diffusion process (also called a solution nitriding process) wherein an austenite stainless steel is treated in nitrogen gas atmosphere in a range of temperatures as high as 1200° C. to cause nitrogen (N) to be absorbed into the surface of the steel and diffused into the solid phase.
However, since such N absorption and diffusion processing is usually performed in relatively high temperature regions of 1200° C. or above, it causes enlargement of the crystal grain in the austenite steel material, resulting in a marked loss of its ductility in contrast to a highly increase in strength thereof due to forming a high concentration solid solution of N.
In addition, it is practically difficult to effectively apply the N absorption and diffusion process to steel plate or pipe materials with a relatively large wall thickness in view of the time required in such process.
Especially, in the case where N absorption and diffusion treated pipe materials are utilized, for instance, as hollow materials like fuel gas storage tank, for fuel cell vehicles (FCVs), with relatively large dimensions, e.g. diameter and thickness, they are required to have not only their own ductility but also strength sufficient thereto; any satisfactory austenite stainless steel material is not achievable as yet.